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It’s easy to get confused about when to prune hydrangeas or how often to water orchids, but understanding certain garden terms can be even more challenging. How can you watch for true leaves to emerge if you don’t know what true leaves are? And is a cotyledon part of a seed, or some kind of dance?

Check out our gardener’s glossary to review some words you’ll often encounter when you’re ready to reach for a shovel or watering can.

Damping off – A disease or condition that causes seedlings to weaken and die. Damping off can also keep seeds from germinating, or result in poor germination. It’s caused by various fungi present in soil and often occurs when there is high humidity and poor air circulation, and/or when seeds are sown too thickly.

To prevent damping off, use a sterile potting mix instead of ordinary garden soil. Use clean, new pots, or sterilize used pots with one part bleach to 10 parts water. When you sow seeds, leave room for good air circulation, and thin as recommended on the seed packet. Water seeds and seedlings from the bottom.

Cotyledon – A leaf that is part of the embryo in a plant seed. It usually becomes the first embryonic leaf or leaves when the seed germinates. Although cotyledons function like leaves, they’re not the same as true leaves. Cotyledons are formed before seeds germinate; true leaves appear after germination.

Bolt – A plant is said to “go to seed” when it rapidly produces flowering stems and seeds as leaf growth slows down or stops. Bolting usually happens in hot weather and results in poor flavor or harvest quality. Lettuce and spinach are examples of plants that bolt when the temperatures rise.

Deadhead – To remove faded flower heads. This encourages a plant to continue blooming, rather than setting seeds.

Cloche – A bell-shaped covering, usually made of glass or plastic, used to protect plants from frost, wind or cold weather. Cloches may also be used to keep garden pests away from plants or maintain proper humidity and temperature.

Harden off – To gradually acclimatize seedlings to outdoor temperatures, winds, humidity and sunlight before transplanting them into the garden. Young plants can be toughened up, or hardened off, by slowly increasing their exposure to outside conditions over a period of a week or two.

Botrytis – A fungus that forms a gray, powdery mold on plants, usually when warm weather follows a period of cool, damp weather. Also called “noble rot,” botrytis may be desirable when growing grapes. It can help dehydrate the fruits, producing more sugar and acidity.

Petiole – The slender stalk that holds a leaf to a stem. A sessile leaf is a leaf that lacks a petiole and grows directly from the stem.

Day-neutral plant – A plant that flowers regardless of how long it is exposed to light. Corn, tomatoes, rice and cucumbers fall into this category. Short-day plants, such as poinsettias, chrysanthemums and Christmas cacti, flower when the days are less than 12 hours long. Long-day plants, which include lettuce, potatoes, asters and California poppies, need more than 12 hours of light to produce flowers. Some plants, such as petunias, flower more heavily when the days are long, but bloom regardless of day length.

Diatomaceous earth – The fossilized remains of ancient, aquatic organisms called diatoms. Diatom skeletons are composed of silica, and the diatomaceous earth, or DE, that is made from the skeletons, is about 85% silica. DE is used as an organic pesticide.

pH – A measure of how acidic or alkaline a substance is; in gardening, pH measures the soil’s acidity or alkalinity. It’s important to know your garden’s pH because you may need to add amendments to help your plants take up nutrients from the soil. Many things can affect pH, including the temperature and amount of rainfall you receive.

Most plants grow well in soil with a pH of 6.0 to 7.0, which is slightly acidic to neutral. You can use a home test kit to determine your pH, or contact your local extension service office. Many extension services will test a soil sample for you and recommend amendments for a small fee. Be sure to tell the extension agent what you plan to grow.

Humus – A black or brown organic substance made up of decomposed vegetable or animal matter, such as decaying leaves, insects, twigs and many other materials. Composted food and garden scraps are human-made humus. This useful substance provides nutrients for plants and helps the soil retain water.

Loam – Loam is a not a type of soil in and of itself; instead, it’s a nutrient-packed combination of sand, clay and silt. Clay and silt help garden soils retain moisture, while sand allows for good drainage and keeps the soil particles from becoming tightly compacted. Loam is usually dark brown or black; it forms a loose ball when squeezed in your hand and crumbles easily.

Hybrid - Hybrids results when two plants are cross-pollinated to produce a plant with the most desirable traits or characteristics from each. Hybrids can be created by plant breeders or by nature. Seeds from a hybrid may be sterile; others will not grow “true-to-type.” That is, hybrid seeds produce baby plants that revert to one of the parent forms used to create them. There are a few exceptions in hybrid seeds, but not many.